Linear motors are used in a variety of applications, including automated positioning systems, robotics, hard drives, semiconductor manufacturing, and electronic assembly. A description of linear motor construction and operation is provided in U.S. Pat. No. 4,839,543, issued to Trilogy Systems Corporation, which is incorporated herein by reference. A position encoder system for positioning linear motors is described in U.S. Pat. No. 6,326,908, also issued to Trilogy Systems Corporation, and also incorporated herein by reference.
Shown in FIG. 1 is an end cross-sectional view, of a conventional assembly for mounting a linear motor. Linear motor coil assembly 1500, which may be a core-less or an iron core motor assembly, is affixed to a carriage plate 1100 by way of bolts inserted into a plurality of bolt holes 1530. Carriage plate 1100 serves to conduct heat away from assembly 1500 and also serves to mount bearing block assemblies 1000 using bolts inserted into bolt holes 1110. Bearing block assemblies 1000 comprise roller bearings or ball bearings 1032 that roll against bearing rails 1030. Bearing rails 1030 are secured to a base plate 1200 by bolts inserted into a plurality of bolt holes 1041. The base plate 1200 may be secured to a fixed structure with bolts inserted into a plurality of bolt holes 1201. Also secured to base plate 1200 by bolts inserted into a plurality of bolt holes 1401 is a magnet support structure 1400, which supports a linear array of magnets 1540. Mounted to the linear motor coil assembly 1500 is a connector assembly (not shown) that enables current to be coupled to the coils of the motor. Also not shown are sensors for detecting the position of the motor as described in U.S. Pat. No. 6,326,908.
Shown in FIG. 2 is a motor core assembly 8500 positioned above a linear array of magnets. The magnets alternate in polarity between north 8541 and south 8542 poles. The magnets are disposed in a linear array upon a magnet support structure (not shown) and separated by spacers 8543. An undesirable force disturbance occurs when the ends of core assembly 8500 traverse over a transition between a north polarity magnet 8541 and a south polarity magnet 8542. The graph 8901 is roughly indicative of the rise and fall, as a function of position, of the magnitude of the force. This cyclical force undesirably acts upon core assembly 8500, a phenomenon known in the art as cogging. There is a need to reduce the cogging force because cogging undesirably interferes with the smooth and efficient transfer of power to the motor.
One prior art method for reducing cogging is to skew the coils with respect to the axis of the magnets or to skew the magnets with respect to the axis of the coils. But skewing results in a loss of motor efficiency. Another prior art method for reducing cogging is to apply shaped magnets, but this too results in loss of efficiency. Also, the manufacture of shaped magnets is more expensive than the manufacture of rectangular magnets.
Thus, there is a need for an anti-cogging method that substantially eliminates cogging, is not expensive to implement, and causes little or no efficiency loss